U.S. patent number 4,495,070 [Application Number 06/442,586] was granted by the patent office on 1985-01-22 for horizontal vacuum belt filter.
Invention is credited to Henri G. W. Pierson.
United States Patent |
4,495,070 |
Pierson |
January 22, 1985 |
Horizontal vacuum belt filter
Abstract
A filter belt is progressed stepwise over successive vacuum
boxes by a guide roller at the forward end of the upper run being
extended from its retracted position to its full line position
while vacuum to the boxes is shut off. A displaceable roller
permits this by taking up any slack in the belt and return movement
of the belt upon retraction of the foremost guide roller is
prevented by a unidirectional arrangement in at least one of the
rollers guiding the lower return run of the belt. Slurry is fed
onto the belt by a first nozzle, optional wash liquor is fed onto
the resultant filter cake by a second nozzle and optional
recirculated wash liquor is fed by a third nozzle all under the
influence of a control which provides for such feed only for a
predetermined period of time during the actual movement of the
belt.
Inventors: |
Pierson; Henri G. W. (Bozeat,
Northamptonshire, GB2) |
Family
ID: |
10518905 |
Appl.
No.: |
06/442,586 |
Filed: |
November 18, 1982 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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317298 |
Nov 2, 1981 |
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Foreign Application Priority Data
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Jan 10, 1981 [GB] |
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8100710 |
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Current U.S.
Class: |
210/139; 210/400;
210/409; 210/196; 210/406 |
Current CPC
Class: |
B01D
11/023 (20130101); B01D 29/09 (20130101); B01D
29/78 (20130101); B01D 29/684 (20130101); B01D
2201/204 (20130101); B01D 33/747 (20130101) |
Current International
Class: |
B01D
11/02 (20060101); B01D 29/09 (20060101); B01D
33/00 (20060101); B01D 33/74 (20060101); B01D
033/32 () |
Field of
Search: |
;210/138,139,140,400,401,783,785,406,409,100,102,768,772,196,137 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Fisher; Richard V.
Assistant Examiner: Millard; Wanda L.
Attorney, Agent or Firm: Ross; Kenwood Flavin; Chester
I.
Parent Case Text
FIELD OF THE INVENTION
This invention concerns a horizontal vacuum belt filter and is a
continuation-in-part of my application Ser. No. 317,298, filed the
2nd day of Nov., 1981, now abandoned.
Claims
I claim:
1. A filter belt assembly comprising in combination:
a filter belt disposed in an endless configuration defining a
substantially horizontal upper run and a return lower run
strategically entrained over a system of frame-mounted rollers, a
slurry supply means for charging a slurry onto the top side of the
upper run,
a wash liquor supply means for feeding a wash liquor onto the top
side of the upper run,
a recirculated wash liquor supply means for feeding a recirculated
wash liquor onto the top side of the upper run,
a vacuum means disposed beneath the upper run for drawing the
liquid content from the slurry and wash liquors through the belt
with the remaining solid content of the slurry being deposited in
cake form upon the top side of the upper run,
a drive means for effecting incremental forward movement of the
upper run relative to the vacuum means,
a variable valved control means operative upon commencement of each
increment of forward movement of the upper run for allowing slurry
charging during a first pre-set time period of the forward movement
and for preventing slurry charging at all other times and also for
allowing wash liquor charging during a second pre-set time period
of the forward movement and for preventing wash liquor charging at
all other times in the obtainment of a thin and even spread of the
slurry on the upper run and an adequate washing thereof, the
control means including a first pipeline loop and a first pump
operative for causing slurry to flow therearound continuously,
a first branch leading from the loop to the slurry supply
means,
a first valve means disposed in the first branch,
a first timer for causing the valve to be opened only for the first
pre-set time period,
a second pipeline loop and a second pump operative for causing wash
liquor to flow therearound continuously,
a second branch leading from the loop to the wash liquor supply
means, and
a second valve means disposed in the second branch, and
a second timer for causing the second valve to be opened only for
the second pre-set time period.
Description
In particular, this filter is of the kind comprising an endless
filter belt or cloth guided to provide a substantially horizontal
upper run onto which is fed a slurry required to be filtered, the
belt or cloth being adapted to be moved forward stepwise so as to
progress successive portions of the upper run over a series of
vacuum boxes which serve to suck away liquid from the slurry and to
leave solid from the slurry deposited on the belt or cloth as
filter cake.
BACKGROUND OF THE INVENTION
In certain known types of horizontal vacuum belt filter, for
example as disclosed in my earlier U.S. Pat. No. 3,870,641 dated
the 11th day of Mar. 1975, the filter belt or cloth, and with it
the filter cake, is displaced intermittently in discrete
increments. In order to permit the displacement, the vacuum system
connected to the vacuum boxes is switched off, and the filter belt
or cloth, with the cake and/or slurry thereon, is displaced in a
discrete increment, after which the vacuum is switched on again,
this sequence of operations being repeated continuously.
In other types of known horizontal vacuum belt filters, e.g. as
disclosed in U.S. Pat. No. 4,038,193 (van Oosten), the filter belt
or cloth, with the filter cake, moves continuously but the vacuum
boxes travel intermittently backwards and forwards therebeneath,
each period of backward travel occurring during a period whilst the
vacuum is switched off.
One of the main reasons for the increased use of horizontal vacuum
belt filters in recent years is that they permit vastly improved
cake washing efficiency compared to rotary drum and other filters.
Nowadays, however, the pharmaceutical and fine chemical industries
in particular tend to demand even more stringent cake washing
requirements. There requirements are not only stringent in terms of
ultimate cake purity but also in terms of the amount of wash liquor
used. The reasons for this are easy to understand. Either the wash
liquor goes to waste and may cause an effluent problem, or, the
wash liquor is an expensive medium in its own right so that
restriction of its use is desirable, or, the final wash liquor is a
valuable product and has to be recovered through multi-stage
evaporation.
Initially, reduced volumes of wash liquor were achieved by
introducing multi-stage counter-current washing, but practical
difficulties soon discouraged this system, further limiting the
amount of wash liquor used. To achieve efficient cake washing, it
is obviously necessary for the wash liquor to cover the entire
width of the cake on the belt. It has been found in practice that
in order to cover a continuously moving slab of filter cake a
minimum volume of approximately 5 to 6 liters (1.10 to 1.32
gallons) per minute per meter (39 inches) width and more
practically something approaching twice that volume is required. In
theory this figure could be reduced by use of fine sprays, but in
practice the spray nozzles simply became blocked or the rate of
flow was insufficient to ensure adequate pressure and distribution
of the liquor issuing from the nozzle. Alternatively, an
overflow-weir system might theoretically provide a small rate of
flow on a continuous basis. However, the laws of hydraulics are
such that the minimum flow required to draw an even film over the
lip of a weir is approximately the above-mentioned amount of 5-6
liters per minute per meter width. In other words, the process
requirements dictated by certain manufacturers meant that the
desired volume of wash liquor was considerably less than the volume
necessary to achieve a hydraulic practicality.
A similar problem arises in relation to the supply of slurry to the
filter belt of a horizontal vacuum belt filter. Most efficient
washing, having regard to the final purity of the filtered product,
is achieved when a thin, even layer of slurry is deposited on the
belt. In practice, however, there is always a minimum and
frequently a maximum speed at which slurry can be pumped through a
pipe. If the velocity of the slurry is too slow, settling out and
blockage of the pipeline occurs and if the flow is too high (as may
occur in a pipe of very small diameter) damage may be caused to the
fragile particles in the slurry and/or pressure may build up to an
unacceptable limit. In practice, therefore, it is extremely
difficult to pump slurry at a rate which, in theory, is highly
desirable to obtain the correct quantity of slurry on the belt.
PRIOR ART
No previously known apparatus has been designed to tackle the
problem of limiting or controlling the flow of wash liquor and/or
slurry to a horizontal filter belt so as to achieve an even thin
spread of slurry and adequate washing with a small quantity of wash
liquor.
The filter apparatus described in the above-mentioned U.S. Pat. No.
4,038,193 includes distribution nozzle for applying slurry or wash
liquor to a continuously moving belt at a continuous rate and
thereby has the abovementioned disadvantage that the amount of
slurry or wash liquor supplied cannot in practice be reduced beyond
a certain limit dependent on hydraulic principles.
U.S. Pat. No. 3,966,610 (Gibbs) describes apparatus wherein liquor
is removed from above the filter cake, thereby improving the
draining rate. This is completely different from the presently
proposed apparatus. U.S. Pat. No. 4,142,971 (Le Fur et. al) and
U.S. Pat. No. 4,276,168 (Bastgen) each disclose moveable rollers
for tensioning a filter belt, but do not disclose a moveable roller
which serves to move the filter belt intermittently, as is proposed
in the present invention. The vibrator described in U.S. Pat. No.
4,243,527 (Leonard) serves to clean a screen through which earth
drilling fluid is filtered and does not agitate a horizontal filter
belt on which slurry or filter cake is deposited as is preferably
required in the present case. Finally U.S. Pat. No. 4,292,173
describes a vertical filtering device having a timer (of an
undisclosed nature) to automate the sequence of events comprising
closing together the layers of the device, supplying slurry to
each, washing the resultant cake in each layer, applying a vacuum,
opening the device and progressing the layers to a cake discharge
position.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a construction of
intermittently-moving horizontal vacuum belt filter in which the
above-discussed problems are overcome.
With this object in view, the present invention provides a
horizontal vacuum belt filter of the kind above referred to having
liquid feed means for supplying liquid (which may be slurry
required to be filtered or may be wash liquor for washing filter
cake on the belt or cloth) to the belt or cloth characterised in
that it comprises a control which is operative, upon commencement
of each forward movement of the belt or cloth, to actuate the feed
means to supply its liquid to the belt or cloth during a
predetermined period of time during its forward movement.
A preferred construction of the belt filter of the invention has
its respective liquid feed means for the slurry and for the wash
liquor, with each feed means having its own control to supply its
slurry and its wash liquor respectively for respective
predetermined periods.
Each control may be variable.
Each control conveniently comprises, the liquid supply means, a
by-pass valve connected to a time delay unit, which may be variable
in its setting, which serves, after a predetermined period of
opening of the valve to an outlet of the liquid feed means, to
switch off the supply to the outlet.
Where the liquid feed means serves to supply the slurry, in order
to control cake thickness the by-pass valve would be fitted in the
feed line to the slurry feed outlet and this by-pass valve is
connected to the time delay unit. This time delay unit then takes
its impulse from the forward movement of the filter belt or cloth
and during a preset time, the valve remains in a position
corresponding to the feeding of slurry to the belt or cloth, until
the desired predetermined period has expired and a known volume of
slurry has been supplied, whereupon the by-pass valve is activated
to enable the slurry to be recirculated. Once the time delay unit
has been set to give a required cake thickness, the frequency of
the intermittent forward stepwise movement of the belt or cloth can
be adjusted by the operator at will without in any way affecting
the cake thickness, which is simply controlled by the time delay
unit. In this manner a completely non-mechanical non-contact system
is, for the first time, made available to permit cake thickness
control.
The same principle applies where the liquid feed means serves to
supply cake wash liquor. As mentioned previously, the normal
problem lies in the difficulty in applying relatively small
quantities, on a continuous basis, in order to achieve complete
coverage. Using the time delay principle as discussed above, again
the by-pass valve will be located in the wash liquor feed system
and this valve will be actuated so that wash liquor is fed to the
belt or cloth for a predetermined period each time when the
cloth/cake moves forward. In this manner high pressures and high
volumes can be used during the period that the wash liquor is being
supplied and thereby complete coverage of the cake can be ensured,
yet the total volume used may be very small, the valve being in a
condition providing for wash liquor flow to the belt of cloth only
for its short predetermined time.
An important feature of the invention is that once the system has
been set, increase or decrease in the frequency of the intermittent
forward stepping movements of the belt or cloth has no effect on
the efficiency of the system which automatically adjusts to the
belt-stepping frequency used at any particular time.
Once a filter cake has been partially washed, any further washing
is often difficult since intimate contact between the cake
particles and the wash liquor often does not occur due to the wash
liquor preferentially travelling through minute channels in the
filter cake which have been created during previous filtration and
washing steps. Accordingly, a further optional refinement of the
filter of the invention lies in the provision of vibratory means to
agitate the belt or cloth, and therewith the filter cake, during
the period when the vacuum is switched off. This shakes the filter
cake up, redistributes the material and re-structures or
re-slurries the cake thereby eliminating preferential channelling
of wash liquor, greatly increasing contact between the wash liquor
and the cake particles and improving the efficiency of the washing
operation.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described further by way of example, with
reference to the accompanying drawing, in which:
FIG. 1 is a diagrammatic side view illustrating a horizontal vacuum
belt filter to which the present invention is applicable;
FIG. 2 is a simplified view comparable with FIG. 1, showing a
slightly modified embodiment;
FIG. 3 is a simplified circuit diagram illustrating a practical
embodiment of control arrangement suitable for the wash liquor
supply or the feed slurry supply in the apparatus of FIGS. 1 and
2;
FIG. 4 is a side elevation of a preferred practical embodiment of a
horizontal vacuum belt filter to which the present invention is
applicable, the supply, drainage and recycling system for wash
liquor being omitted when compared to FIG. 1 for the sake of
clarity;
FIG. 5 is a rear view of the moveable roller in the direction of
the arrows V--V in FIG. 4;
FIG. 6 is a detailed cross-section of a vibrator and vacuum tray
which is included in the preferred embodiment of the filter of the
invention;
FIG. 7 is a schematic diagram of a timercontrolled wash liquor and
slurry supply system of a modified embodiment of the invention;
and
FIG. 8 is an enlarged schematic diagram illustrating how
intermittent slurry supply may be achieved according to the
invention.
Referring firstly to FIGS. 1, 2 and 4, a practical embodiment of
the horizontal vacuum belt filter conforming to the invention
comprises an endless belt or cloth 10 guided around rollers 11, 12,
13, 14, 15, 16, 17 and 18 to provide an operative upper run 19
which extends over a succession of vacuum boxes 20, 21, 22, 23 and
24. The rollers are typically manufactured out of 6 inch diameter
steel tubing with a central shaft of about 21/2 inches diameter.
None of these dimensions are critical beyond minimum strength
requirements. The rollers 11 to 18 are, of course, in practice
supported by a framework of uprights and transverse cross-beams, as
shown in FIG. 4.
Roller 11 located at the downstream or front end of the upper
operative run 19 of the belt 10 is the main drive roller and the
extremities of its shaft fit in plummer block ball bearings which
are mounted in a yoke 27. This yoke 27 is moveable disposed on top
of slides 28, as shown in FIG. 5 and is attached to the piston rod
29 of a pneumatic or hydraulic ram 43 operative to push the roller
11 forwards or backwards so as to reciprocate between an extended
position by roller 11 shown in solid lines and a retracted position
11a shown in broken lines in FIGS. 1 and 4.
The guide roller 16 is biased resiliently in the direction
indicated by arrow 25 in FIG. 1 or in the direction indicated by
the arrow 26 in FIG. 2, to take up slack which would otherwise
arise in the belt or cloth 10 when the foremost guide roller 11
moves to its retracted position 11a. For example, as shown in FIG.
4, the extremities of the shaft of the roller 16 may be located in
respective slots 44 in opposing frame members 49 and have a
suspended weight 45 connected thereto by cord or wire. Thus the
roller 16 moves relative to the slots 44 between position 16a,
shown in dotted lines to take up slack in the belt 10 when the
roller 11 is retracted, and the position of the roller 16 shown in
solid lines when the roller 11 is extended and the weight moves
correspondingly between lower position 45a and upper position
45.
At least one of the guide rollers 13, 14, as shown in FIGS. 1 and 2
is fitted with an appropriate mechanism which restricts its
direction of rotation such that the belt or cloth 10 may travel
only in the direction indicated by the arrow 60. FIG. 4 illustrates
how this may be put into effect in practice, both rollers 13, 14
incorporating pawl and ratchet mechanisms 46 thereby permitting
belt movement only in the desired direction. Also, in the
illustrated embodiment, the rollers 13, 14 form part of a secondary
drive arrangement in the lower run of the belt 10. The remaining
rollers 12, 15, 17 and 18 are fixed deflection and support rollers
and serve only to guide the belt 10 in the required path. Further
belt tracking devices 47 are shown in FIG. 4.
Intermittent forward motion of the filter belt 10 is achieved in
the following manner. Starting from the condition in which the
parts are in the dotted line condition, i.e. the main drive roller
is in its retracted position 11a and moveable guide roller 16 is in
position 16a, a vacuum is applied to vacuum boxes 20 to 24 so that
the operative upper run 19 of the belt 10 cannot move. Slurry
supplied to the upper run 19 from feed inlet 30 is subjected to the
vacuum and is dried to remove its liquid content leaving behind a
filter cake on the upper run 19. After a predetermined time, the
vacuum is switched off and the upper run 19 of the belt is released
for movement. Piston rod 29 of ram 43 extends to its solid line
position, and, since the ratchet devices 46 on the rollers 13, 14
will not permit movement of the belt 10 in the reverse direction to
the arrow 60, the movement of the piston rod 29 draws a length of
belt out of the bight 48 between the guide rollers 15, 17 into the
upper run 19. The roller 16 is thus drawn into its solid line
position 16, at the extreme right of FIG. 4.
Once the aforesaid operations are completed, the vacuum to the
vacuum boxes 20 to 24 is switched on again so that once again the
operative upper run 19 is prevented from moving. The piston rod 29
is retracted to its dotted line position and the resultant slack is
drawn into the lower return run of the belt 10, via the nip
provided by the rollers 13, 14, by movement of the roller 16 to the
left in the FIGS. 1 and 4 under the influence of the weight 45 to
the dotted line position 16a. The bight 48 is thus restored to its
original length.
The above cycle of operations is performed repetetively and as a
result the slurry is converted into solid filter cake with
progressively reduced liquid content as it moves along the upper
run 19 of the belt 10. Eventual dry product separates from the belt
either at the roller 11 or the guide roller 12.
The combined effect of the roller 16, the movement of the foremost
guide roller 11 and the unidirectional effect of one or both of the
rollers 13, 14 is to ensure that each extension of the foremost
guide roller 11 causes a stepping movement of the upper run 19 of
the belt or cloth 10.
As intimated, a feed slurry supply nozzle or weir box 30 serves, as
will be described later, to supply to the belt or cloth 10, just in
advance of the guide roller 18 on the upper run 19 above the vacuum
box 20, a slurry required to be filtered using the apparatus.
As shown in FIG. 1, a first wash liquor nozzle 31 is disposed above
the upper run 19 above the vacuum box 21 and a second wash liquor
nozzle 32 is correspondingly disposed above the vacuum box 22.
The vacuum box 20 is connected to a vacuum receiver 33; the vacuum
box 21 is connected to a vacuum receiver 34; the three vacuum boxes
22, 23 and 24 are connected together and to a common vacuum
receiver 35. All three vacuum receivers 33, 34 and 35 are
connected, by their upper parts, to a common vacuum pump 36.
Outlets 37 and 38 from the lower parts of the vacuum receivers 33
and 34 are connected to respective pumps 39, 40 for liquid
therefrom to be directed away from the apparatus, e.g. for
subsequent treatment or for discharge. On the other hand outlet 41
from the common receiver 35 leads to a pump 42 whose outlet is
valved to enable its output to be directed away from the apparatus
(as above discussed) or to be directed to the first wash liquor
nozzle 31.
A pump 50 serves to supply washing liquid to a belt or cloth
washing nozzle 51 disposed above the belt or cloth 10 between the
guide rollers 12 and 13 so that washing liquid sprayed onto the
belt or cloth 10 by the nozzle 51 washes all residue from the belt
or cloth 10 into a tank 52.
For slurry deposition and filtration, with the roller 11 in the
withdrawn or retracted dottedline position at 11a, the slurry is
supplied by way of the nozzle or weir box 30 whilst the belt or
cloth 10 is stationary and vacuum is applied to the vacuum boxes 20
to 24 by the pump 36. Liquid from the receivers 33, 34 and 35 is
drawn away as described. During the slurry feed, the nozzle or weir
box 30 may be vibrated and/or displaced either transversely or
longitudinally of the belt or cloth 10, as may be appropriate for
the material being treated. After a predetermined period of time, a
mechanism (not shown) is triggered to switch off the vacuum to the
vacuum boxes 20 to 24, and to actuate the hydraulic or pneumatic
ram 43 to cause the roller 11 to be moved to the extended full line
position in FIG. 1, thereby stepping the upper run 19 of the belt
or cloth 10 forward by one step in the direction of the arrow 60.
The vacuum is then switched on again, whilst the ram retracts the
roller 11, so that filtration continues, with solid material from
the slurry building up as filter cake indicated diagrammatically at
62 on the operative upper run 19 of the belt or cloth 10.
For washing the filter cake 62, an appropriate wash liquor is
supplied to the nozzle 32 from which it sprays onto the filter cake
62 through which it passes to be taken up by the vacuum boxes 22,
23 and 24. According to the setting of the valve controlling the
output of the pump 42, the liquid from the boxes 22 to 24 can
either be directed away from the apparatus or can be recirculated
to the first wash liquor nozzle 31.
Turning now to FIG. 3, this figure illustrates diagrammatically, a
control arrangement. A respective one of these may be provided for
controlling the slurry supply to the nozzle or weir box 30, and/or
the wash liquor supply to the second wash liquor nozzle 32 and/or
the recirculated wash liquor supply to the first wash liquor nozzle
31. The figure shows the control arrangement applied to the nozzle
32 and as illustrated includes a switch 70 which is arranged so as
to be actuated simultaneously with the triggering of the ram 43
which displaces the roller 11. Upon such triggering, the switch 70
supplies an impulse to a control valve 71 which serves, through a
timer 72, to open a shut-off valve 72 which controls the supply of
wash liquor to the nozzle 32. Accordingly, wash liquor is supplied
to the nozzle 32 only for a predetermined period of time, as set by
the timer 72, which is adjustable, which thereupon is operative to
actuate the control valve 71 and shut-off the wash liquor.
The timer 72 may, of course, be any locally commercially available
timer. It must, however, be able to re-set itself automatically, be
adjustable for time interval and be capable of making a positive
contact after the pre-set time interval has expired. By setting the
timer 72, one can provide for a predetermined amount of wash liquor
to be supplied to the filter cake 62, upon each stepping movement
of the belt or cloth 10, and this wash liquor feed will vary
automatically and in proper proportion with an increase or decrease
in the frequency of the stepping movement of the belt or cloth 10
and does not need individual adjustment upon such increase or
decrease being effected.
A comparable arrangement can readily be incorporated in the feed
line for recirculated liquor supplied to the first wash liquor
nozzle 31.
Most important, however, is the fact that the slurry supply to the
nozzle or weir box 30 can also incorporate a comparable
arrangement, and this will, once it is appropriately adjusted,
ensure that the slurry feed to the belt or cloth 10 is directly
related to the progressing or stepwise movement of the belt or
cloth 10 regardless of any adjustment of the frequency of movement
thereof.
Using the above-described system, the slurry or wash liquor can,
for example, be pumped or caused to flow in a continuous loop at a
rate which is ideal for the pipeline, the pump system and the type
of material, a branch leading from the loop having a valve which is
opened on a timed basis. As long as flow through the loop is kept
constant, the amount of slurry or liquid distributed through the
valve can be accurately measured simply by adjusting the time
period during which the valve is open. For example, if a volume of
5 liters (1.10 gallons) per minute is required to be evenly spread
across a filter belt, the slurry or liquid in the loop could be
circulated at a suitable rate, e.g. 20 liters (440 gallons) per
minute, and the branch valve only open for 15 seconds in every
minute, thereby releasing the required 5 liters (1.10 gallons) per
minute although the actual rate of flow through the system is much
higher.
This system eliminates all the flow and distribution problem set
out in the introduction hereto and is much simpler and trouble free
than systems wherein flow is controlled by flow-meters, variable
speed pumps and compensating valves. Moreover, this system is only
practical with an intermittently moving filter belt since, each
time the belt moves or remains stationary a large quantity of
liquid can be introduced for a short period. (This would obviously
not be possible with a continuously moving belt).
Further examples of the timer-and-valve-controlled wash liquor and
slurry feed system proposed by the invention are illustrated
diagrammatically in FIGS. 7 and 8. When the guide roller 11 moves
to the left to its extended position, it trips the switch 70 (which
may be electric, pneumatic or hydraulic) to actuate a master timer
74 which in turn actuates the individual timers 72 to open or close
valves 73 for a pre-set time. Each timer 72 may, of course, if
required, open or close respective valves 73 for different pre-set
periods. The master timer 74 is a safety device causing the whole
circuit to cut off after a maximum time delay. Each time the roller
11 is extended, exactly the same sequence will occur so that as a
portion of belt or cloth 10 and the cake thereon is moved stepwise
forward, wash liquor is introduced for a certain time and slurry is
introduced for a given time to replenish that which has moved
forward.
As hereinbefore set forth, the efficiency of cake washing is
considerably enhanced if the filter cake is shaken up and
re-slurried during the washing procedure. There are various methods
of vibrating a horizontal filter belt and the cake deposited
thereon, but the preferred means in the present case are vibrators
attachable to the underside of one or more of the vacuum boxes or
trays 20 to 24 so as provide vibratory trays which cause vibration
of the overlying portion of the belt 10 and thus re-structuring and
re-slurrying of the cake formed thereon. A typical arrangement of
vibratory tray is illustrated in FIG. 6. A vibrator 53, which can
be any commercially available vibrator, e.g. electric or
air-operated, as long as it has variable frequency is bolted onto
the underside of one of the vacuum trays 20, which is mounted upon
rubber pads 54 to as to be capable itself of vibrating.
Most filter cakes are thixotropic which means that they will
display flow properties if subjected to mechanical action. Since
the filter cake (designated by reference numeral 55) lies firmly
against the tray 20 when a vacuum is applied thereto, when the tray
20 vibrates, the vibrating action of said tray 20 is imparted to
the cake 55 causing it to flow together. In this way, any cracks or
channels in the cake will disappear so that further dewatering can
take place and thorough cake washing be effected.
The invention is not confined to the precise details of the
foregoing example, and variations may be made thereto. Naturally it
is not essential that the control should be applicable only to
machines having means for supplying both slurry and wash liquor and
it can be employed in cases where only a slurry supply or a wash
liquor supply is present.
* * * * *